Hydrogen may act as an energy source or carrier for many applications, including as a fuel for transportation. Transportation applications, such as for cars, require compact, and preferably lightweight, hydrogen storage. Hydrogen has good energy density by weight by typically poor energy density by volume, so compact storage requires pressurization of the hydrogen.
Cryo-compression uses cold hydrogen stored in tanks that can handle up to 350 bars (5000 pounds psi) of internal pressure. As the hydrogen warms up from heat transferred in from the environment, the tank has more time before it needs to vent the hydrogen. In transportation applications, the vehicle has used enough hydrogen by that time to keep the pressures below the venting limit.
Current cryo-compressed hydrogen tanks with vacuum insulation current satisfy DOE's 2017 targets for energy density. However, the thermal insulation performance remains satisfactory for only 3 weeks or so, as the vacuum quality degrades, as discussed by S M Aceves, et al., in the International Journal of Hydrogen Energy, vol. 38 (2013), pp. 2480-2489.
These tanks typically have an architecture with an inner pressure vessel with a pressure resistance up to 350 bar, made of aluminum wrapped with carbon-fiber reinforced polymer (CFRP). A vacuum and numerous sheets of high reflective metalized plastic provide high performance thermal insulation. Currently no material exists to replace the high vacuum without an unacceptable decrease in the volumetric energy density. It has been determined that the main reason for pressure increase in the vacuum liner is the outgassing of volatile hydrocarbon-based resins present in the CFRP epoxies [Reference: S. Aceves et. al. Project # ST003, Hydrogen AMR (2010)]. Therefore, it appears that the only viable solution to extend the vacuum lifetime is to mitigate the outgassing of the epoxy resin from the CFRP tank walls.
One possible option would be to use low outgassing epoxy resins, such as those for space applications, for fabrication of a CFRP tank with reduced outgassing. CFRP typically contains 40% epoxy resin. Low outgassing resins are generally very viscous and cost more than the carbon fibers themselves, and are therefore not suitable for fabricating a cost-effective CFRP tank. A gas barrier coating may protect the vacuum exposed surface of the CFRP wall and may represent a viable, low-cost solution. Current gas barrier approaches have resulted in expensive, ineffective or difficult to implement gas barriers. For example, a UV-cured polymer coating actually result in increased outgassing. Conventional gas barrier polymers such as EVOH (ethylene vinyl alcohol) and oriented PET (polyethylene terephthalate) require high temperature extrusion, typically over 250° C., which is incompatible with CFRP processes. It is difficult to adhere metal foils onto large CFRP surfaces without defects and vacuum deposition of metallic gas barriers costs too much for large surfaces.